1. Field of the Invention
The present invention relates to apparatus for the measurement of linear displacements. More particularly, the invention relates to optical apparatus which is useful for high accuracy linear displacement metrology using interferometry.
2. The Prior Art
An interferometer is a basic instrument for most high accuracy displacement measurements in dilatometry, material stability studies, the machine tool industry, and in the semiconductor fabrication industry. One type of interferometer representative of the current state-of-the-art is the differential plane mirror interferometer which measures the optical path length changes between two external mirrors and which is described in R. R. Baldwin and G. J. Siddall, "A double pass attachment for the linear and plane interferometer," Proc. SPIE, Vol. 180, pp. 78-83 (May 1984) and in commonly-owned U.S. Pat. Nos. 4,693,605 and 4,752,133 issued Sep. 15, 1987 and Jun. 21, 1988, respectively.
A limit on the accuracy of a heterodyne interferometer is the periodic, i.e. cyclic, error caused by leakage of the frequency components in the beamsplitter, see for example, N. Bobroff, "Residual Errors in Laser Interferometry from Air Turbulence and Non Linearity," Applied Optics, Vol. 26, pp. 266-2682 (July 1987) and C. M. Sutton, "Non-Linearity in Length Measurement Using Heterodyne Laser Michelson Interferometry," J. Phys. E: Sci. Instrum. Vol. 20, pp. 1290-1292 (1987).
For differential plane mirror interferometers of the prior art, the magnitude of this cyclic error ranges from .+-.1 to .+-.10 nanometers.
While commonly-owned U.S. Pat. No. 4,950,078 issued Aug. 21, 1990 discloses a high accuracy differential plane mirror interferometer, it does not solve all of the problems which are solved by the present invention which has improved utility for the high accuracy measurement of linear displacement of stages used with probes.